The sterile-male-release program for the control of the New World screwworm, Cochliamyia hominivorax has been an enormous success; nevertheless, a resurgence of cases in Texas has occurred recently, and recalcitrant local populations that have defied eradication efforts have been encountered. Workers at the University of Texas, Austin, have recently suggested that the screwworm is not one species but, rather, a cluster of sibling species; such clusters of morphologically indistinguishable species are well known in Drosophila (flies), Anopheles (mosquitos), Rana (frogs), and other organisms. The possibility that the screwworm actually comprises several sibling species bears heavily on the sterile-male eradication program. Under the present proposal, sibling species of Drosophila would be subjected to a simulated sterile-male control program in the laboratory. (Screwworm flies, because they are a dangerous pest, cannot be studied outside a legally authorized facility.) Parameters that can be varied experimentally include: the degree of sexual isolation exhibited by the sibling-species pairs, the relative proportions of the sibling species, the ratios of released "sterile" flies to the "natives," and the densities of flies in the laboratory mating chambers. These studies should be useful in understanding the responses of screwworms in the field when challenged with overwhelming numbers of sterilized flies. The importance of the study is suggested by (1) the Latin name of the screwworm and (2) the $10-$20 million which are spent annually on screwworm control.